It is known that the cooling of a heat source enclosed in a pressure container--for example, a reactor core delivering decay power--can be secured for a relatively long time by filling the container with water. The power developed in the heat source is then absorbed when water is evaporated, the resulting steam being exhausted from the container via pressure relief means. However, if a rupture occurs in the lower part of the pressure container, water may leak out therethrough as liquid before its steam generating heat is utilized for cooling, the cooling ability thus being lost.
In a means according to the invention, the outer container may be designed for a pressure considerably lower than the working pressure of the inner container. The reason for this is that the water leaking out into the outer container is allowed to flow into the free air through a tube connection, namely into the open space of the evaporation pool, which is located with its bottom totally above the pressure vessel.